The present application relates to ballasts, and power supply circuits for gas discharge lamps. It finds particular application for use with current fed instant and/or rapid start electronic ballasts or power supply circuits and will be described with particular reference thereto. It is to be appreciated, however, that the present application is also applicable to other inverter circuits, and is not limited to the aforementioned use.
In the late eighties, and early nineties, the lighting industry began to make a shift from passive power and harmonic correction circuits to active power correction and harmonic circuits in the form of active pre-regulators for use in conjunction with electronic lamp ballasts. An advantage of active power factor and harmonic correction via active pre-regulators is that bus voltage variation can be virtually eliminated even though there are still voltage variations on the input line. The visible effect of this change is less variation in lumen output, that is, lamps connected to active pre-regulator circuits exhibit steadier intensities than lamps connected to circuits without active pre-regulation.
While the use of active pre-regulators has provided improved performance in certain areas, new problems have arisen when these pre-regulators are put into operation with rapid and/or instant-start ballasts or power supply circuits. Particularly, systems employing active pre-regulators require a significant amount of time to reach steady state operating conditions during start-up. This may result in undesirable operating conditions for the gas discharge lamps when the less than steady state operating voltages are passed through the converter section during this transient start-up condition.
During normal operation, which is a steady state condition, the active pre-regulator will provide a pre-determined DC voltage output, whose value will be dependent on the circuit design and/or lamp being driven, but in many instances may be up to a 500 V DC output. During the transient start-up condition, the output will be substantially below the desired steady state voltage conditions. Therefore, when operating in rapid and instant start modes the voltage supply will not be at steady state, and may result in an undesirable effect of unacceptable “preheat” or glow periods at this lower voltage. Instant-start lamps are typically specified to be operated in a glow discharge mode for a very short time period, approximately for no more than 100 milliseconds. This is a requirement since longer “preheat” periods will act to shorten lamp life due to excessive electrode erosion during these glow discharge conditions. Additionally, when operating in low voltage (i.e. non-steady state conditions), undesirable visible phenomena such as lamp flickering may occur. Therefore, it is considered desirable to delay the start-up operation of an electronic ballast for instant-start type fluorescent lamps until a pre-determined DC bus voltage has been substantially reached.
One particular attempt to address this issue is set forth in U.S. Pat. No. 5,177,408 to Marques which issued Jan. 5, 1993. This patent disclosed a time delay circuit of an electronic ballast for “instant-start” type fluorescent lamps of the type having an electronic converter powered by an active electronic pre-regulator. The inverter is described as an inductive-capacitive parallel-resonant push-pull circuit or other type of current-fed power-resonant circuit. The start-up circuit may be either a resistor and Zener diode or a resistor, capacitor, and diac network programmable uni-junction transistor circuit connected between the pre-regulator output and an oscillation-enabling input of the inverter. The active electronic pre-regulator is designed so that it takes a pre-determined start-up time to reach steady state operating conditions. This delay device is connected between the pre-regulator and the converter.
Drawbacks to the above disclosed design exist. For example, to minimize design and development cost, to lower the number of different products (i.e. SKUs), to simplify inventory control, and to address global market needs, ballasts or power supply circuits having universal input capabilities have become a key selling point. In theory, a device is considered a universal input device if it is capable of operating cooperatively with the various standardized line voltages supplied in different parts of the world. For example, the standard line voltage in the United States is 120 V, in China it is 220 V, and in Europe, 230 V. A universal device would also preferably be able to operate with industrial line voltages which is currently 277 V in the United States.
The aforementioned U.S. Pat. No. 5,177,408 is, however, dependent on the input line voltage to obtain its time delay. This means to obtain a pre-determined time delay, it would be necessary to take into consideration the line voltage with which the device will be operating. Such a device would not therefore be considered a universal input ballast or power supply. Particularly, if a unit were used with a 120 V input line, the time delay would be different than if that unit were receiving a 230 V input line. Thus, this approach does not take full advantage of active power factor correction control.